[3.13] gh-143754: Modernize Tkinter docs (GH-143841) (GH-144033)

Use more relevant terminology instead of "master"/"slave" widgets where possible.
(cherry picked from commit 813fc7a291c98654c27f2b5d8f9afa8e53b066a6)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>